Arenium ions ipso position

We have discussed orientation in the case of monosubstituted benzenes entirely in terms of attack at the ortho, meta, and para positions, but attack at the position bearing the substituent (called the ipsoposition ) can also be important. Ipso attack has mostly been studied for nitration. When NOj attacks at the ipso position there are at least five possible fates for the resulting arenium ion (13). 

Product formation was interpreted in terms of transalkylation of substituted triphenylmethanes. Protonation at the ipso position of the substituted phenyl ring to form arenium ion 64 followed by the C—C bond breaking yields the diphenylmethyl cation, which alkylates benzene or is stabilized by hydride transfer (Scheme 5.30). The protonated intermediate 64 is highly unstable when the ring has an electron-withdrawing substituent. Consequently, its transformation is extremely slow and the primary product triphenylmethane can be isolated. 

Arenium ions possessing a ring fragment of the CHR type or ortho(para)-positioned OH-, NHR- and CHR-type substituents mainly react with rising nucleophilicity of the medium by loss of the proton, as marked earlier. In some cases the Iok of the proton from the substituent competes with the addition of a nucleophile to the ion. For example, arenium ions formed by ipso-addition of N02 capture an acetate-anion (nitroacetoxylation 2-665)j. 

Reaction at the ortho position generates arenium ion 62 with three resonance contributors within the six-membered ring, and if X has unshared electrons, there is one extra contributor with the positive charge on the ipso carbon (C=X+), as shown. Similarly, reaction at the para position generates arenium ion 63, which also has three resonance contributors and one extra with a positive charge on the ipso carbon (when X has unshared electrons). These two intermediates contrast with reaction at the meta position, which gives arenium ion 64, which has three resonance contributors, none of which has a resonance contributor with a positive charge on the ipso carbon. 

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